Hydrogel films (which usually contain greater than 50% water) are useful in many medical applications due to their hydrophilic, or water loving nature, and their ability to act as a conductive member interfacing with the skin of a patient (electrode applications). Typical medical applications involving the use of hydrogels include contact lenses, wound dressings, transcutaneous electrical nerve stimulator (TENS) units, electro-surgical units (ESU's), EKG/EEG applications, iontophoresis, and artificial muscles, artificial organs, and prosthetics. The most widely used polymer in hydrogel formation, especially in medical applications such as implants, blood bags, and syringes, is poly(hydroxyethyl methacrylate).
While there are a variety of hydrogel compositions taught in the prior art which are utilized in medical applications, many of these compositions result in hydrogel films with significant disadvantages. Among the more significant disadvantages are issues associated with monomer-based hydrogel solutions and current hydrogel film production methods.
For example, many of the prior art compositions consist predominantly of a monomer and water solution. Associated with these unpolymerized monomers are strong odors which may be reduced after polymerization. However, because the monomers are not 100% converted, generally about 50% conversion takes place, some odors may remain. Also, these unpolymerized monomers may migrate to the surface of the hydrogel film and may be toxic.
Many prior art hydrogel compositions in current production are are cast and sold as films with release liners and are not suitable for screen printing. Typical hydrogel compositions disclosed in the prior art are applied in a sheet via a non-screen printing technology. The ability to screen print hydrogel compositions requires particular viscosity and shear thinning attributes. These attributes, which are required for screen printability, allow the ability to form a desired pattern and placement of desired dimensions of hydrogel composition in a precise manner. Prior art compositions are limited in ease of hydrogel film formation and processed hydrogel film thickness due to the rheology, shear thinning, and viscosity characteristics of the hydrogel composition. The rheology, shear thinning, and viscosity characteristics of the prior art compositions may lead to slower process times, extensive yield loss, and minimal control over ultimate hydrogel film thickness.
As discussed above, numerous prior art patents exist which disclose hydrogel compositions and medical electrodes utilizing the hydrogel films formed from those compositions. The following illustrate the state of the prior art.
U.S. Pat. No. 5,540,033 to Fox et al., teaches a method for producing a sterile packaged adhesive hydrogel product by preparing an aqueous mixture of water, a polymer which can be cross-linked by radiation (electron beam not UV light) to form a hydrogel, and a cross-linking inhibitor in an amount sufficient to retard the cross-linking of the polymer when the mixture is exposed to radiation; providing the mixture in a predetermined shape or configuration representative of a hydrogel product; enclosing the shaped mixture in a sealed package; and subjecting the package to a dose of radiation sufficient to simultaneously cross-link and sterilize the mixture to provide a sterile packaged adhesive hydrogel product. Fox et al., teaches a providing step which comprises casting the mixture onto a substrate in the desired shape and at a thickness of between about 20 and 100 mils. A scrim is typically applied to the cast mixture when the thickness of the cast mixture is greater than about 25–30 mils.
U.S. Pat. No. 5,868,136 to Fox et al., discloses an electrode providing electrical contact with a patient's skin comprising; a conductive member including means for connection to an external electrical apparatus; and means for electrically interfacing to said patient's skin being electrically and mechanically connected to said conductive member, said interfacing means being a non-liquid film and which comprises an electrically conductive organic polymer plasticized with a polyhydric alcohol with said organic polymer being derived from a monomeric mixture comprising from about 15 to 30 pph acrylic acid, 0.5 to 30 pph N-vinylpyrrolidone and 0.01 to 2 pph of a crosslinking agent and from about 0.5 to 8 pph of a thickening agent comprising a N-vinylpyrrolidone/acrylic acid copolymer. Fox et al., disclose an interfacing film thickness in a range 20 and 100 mils.
U.S. Pat. No. 5,622,168 to Keusch et al., teaches a highly conductive hydrophilic gel comprising a uniform aqueous solution of a crosslinked water-soluble polymer, an amount of a water-soluble electrolyte effective to reduce the transverse electrical resistance of said aqueous mixture to an impedance at 60 Hz less than about 1,000 ohm, which hydrophilic gel also contains a humectant in an amount effective to retard the drying of the conductive hydrophilic gel when it is exposed to the atmosphere while being used. A physiological electrode adapted for providing electrical contact with a surface of a sentient creature and comprising a sheet of the conductive viscoelastic hydrophilic gel. Keusch et al., teaches the use of the hydrophilic gel in an electrode by the following process: 1) casting the aqueous mixture to form a sheet-like configuration (liquid film thickness of 0.1 to 2 mm before crosslinking); 2) subjecting the liquid film to a dose of high energy radiation sufficient to convert said film into a solid gel; 3) optionally cutting the hydrogel sheet to the desired size and shape for use as a conductive element; and 4) placing the solid gel onto the electrode.
WO 97/02811 to Abraham et al., discloses a hydrogel patch, comprising: (a) a hydrophilic compound which forms a gel in the presence of water, which compound is present in an amount of about 4% or more by weight based on the weight of the hydrogel; (b) water in an amount of about 95% or less based on the weight of the hydrogel; (c) and enzyme capable of catalyzing a reaction; and (d) electrolyte. Abraham et al., teaches the casting of the hydrogel composition via a Gardner knife.
WO 00/45698 to Heard et al., discloses an electrode providing electrical contact with a patient's skin which includes a conductive member adapted for connection to an external electrical apparatus, a non-liquid film for electrically interfacing to the patient's skin, the non-liquid film being electrically, and mechanically connected to the conductive member. The non-liquid film includes an electrically conductive organic polymer plasticized with a polyhydric alcohol with said organic polymer being derived from a monomeric mixture comprising from about 15 pph to 30 pph acrylic acid, 0.5 pph to 30 pph N-vinyl pyrrolidine and 0.01 pph to 2 pph of a crosslinking agent. The monomeric mixture may further comprise from about 0.5 pph to 8 pph of a thickening agent selected from the group consisting of N-vinyl pyrrolidine/acrylic acid copolymers and N-vinyl pyrrolidine/vinyl acetate:
Considering the disadvantages of the prior art detailed above, a need therefore exists for a novel hydrogel composition and process which overcomes these disadvantages. Therefore, an objective of this invention is to provide a screen-printable hydrogel composition and process which allows for the ability to form a desired pattern and for placement of desired dimensions of the gel directly onto a substrate with little to no yield loss and control over the ultimate thickness. A further object of the invention is to provide a hydrogel composition which does not have a strong odor prior to processing.